Peter Sobieraj

Today I was fighting with strange error.

 
SELECT
  -- ...
  (
  (SELECT SUM(t.v)
   FROM tab1 t
   )
  +
  (SELECT SUM(t.v)
   FROM tab2 t
   )
  ) AS x
-- ...
 

Something like that give me NULL in x in every row even if there was a lot of records in tab1. Here is why: If no rows match then SUM return NULL, not 0. And another bad think is NULL + 1 = NULL. So to correct that I need to do like this:

 
SELECT
  -- ...
  (
  IFNULL((
   SELECT SUM(t.v)
   FROM tab1 t
   ),0)
  +
  IFNULL((
   SELECT SUM(t.v)
   FROM tab2 t
   ),0)
  ) AS x
-- ...
 

Now it work. I hope it will help you.